Not Applicable.
Not Applicable.
(1) Field of the Invention
This invention relates to a process for the removal of silicon and heavy metals from aqueous streams.
(2) Description of Related Art
It is known in the water treatment art to utilize polyaluminum salts such as polyaluminum chloride, polyaluminum sulfate, and polyaluminum chloride sulfate as flocculating agents for the treatment of both potable and waste water. For example, in U.S. Pat. No. 3,544,476 the use of a polyaluminum chloride is described as a coagulant. These aluminum salts have also been utilized in combination with an effective amount of a polymeric flocculant such as a polyacrylamide, copolymers of acrylic acid and acrylamide, and polymethacrylamide in the treatment of circulating water from a paint spray booth system, as described in U.S. Pat. No. 5,250,189. A method of clarifying waters containing turbidity causing components utilizing a low basicity polyaluminum chloride in combination with a water soluble cationic polymer is described in U.S. Pat. No. 4,655,934, and a water treatment composition containing a polyaluminum salt and polyvinyl alcohol is described in U.S. Pat. No. 4,795,585.
Industrial waste waters discharged from metal refinery works, plating shops, metal processing shops, etc., generally, contain not only heavy metals such as cadmium, chromium, manganese, mercury, etc., but also amounts of iron, titanium, zinc, and silicon. To discharge such waste waters without subjecting them to purification is destructive of the environment. Accordingly, such waste waters are treated by means of a flocculation process.
In the electrochemical art electrolyte is often recirculated for further reaction through an electrochemical cell line after the desired reaction product is drawn off. Accordingly, heavy metal impurities upon such recycling tend to build up in concentration in the electrochemical cell and have a deleterious effect upon the electrodes in that the heavy metal impurities tend to deposit upon the surface of the electrodes which results in a necessary increase in electrical power to overcome the effect of the metal impurity coating.
Heretofore, it has been conventional in the production of sodium chlorate to pass the recirculated electrolyte containing silicon and heavy metal contamination through an ion exchange column containing a resin suitable for removing heavy metals and silicon contamination therein. While this method of removing silicon and heavy metals from an electrolyte may be suitable for certain electrochemical processes, it is unsuitable for a process in which the electrolyte contains an oxidizing agent such as sodium chlorate. This is because it is known that certain oxidizing agents are reactive with the resins utilized in the ion exchange column, in addition, certain of the heavy metals may act as catalysts for the oxidation reaction between the sodium chlorate and the ion exchange resin resulting in the occurence of a fire or even an explosive reaction. Accordingly, it is particularly important to avoid the removal of silicon and heavy metal contamination in an electrolyte prior to recirculating the electrolyte back to the electrochemical cell lines for further production of sodium chlorate.
Sodium chlorate is conventionally produced by the electrolysis of an aqueous solution of sodium chloride in an undivided electrolytic cell. The extent of electrolysis is controlled to produce a product from the cell in which the sodium chlorate and sodium chloride have a desired ratio usually in the range of 1:1to about 20:1 and, preferably, in the range of about 2:1 to about 15:1. Generally, the aqueous solution obtained after electrolysis is processed to crystallize out the sodium chlorate and the aqueous solution remaining, the mother liquor, is recycled to the electrolytic cell utilizing make-up sodium chloride to produce additional sodium chlorate. It is this aqueous solution which must be purified to remove silicon and heavy metals m order to maintain the electrical efficiency of the electrolytic cell. In addition to contamination with silicon and heavy metals, the electrolyte, also known as cell liquor, contains significant amounts of chromate ions. The removal of chromate ions from the sodium chlorate cell product before the sodium chlorate is used in the production of chlorine dioxide for use in the bleaching of chemical cellulosic pulps is set forth in U.S. Pat. No. 5,211,853, assigned to Huron Tech Corp, Delco, N.C.
In accordance with the present invention, there is provided a procedure for removal of silicon and heavy metals from an electrochemical process for the production of sodium chlorate. Altematively, the procedure is applicable to the removal of silicon and heavy metal contamination from aqueous streams. In the preferred process for removal of silicon and heavy metals from an electrochenical process for the production of sodium chlorate, the mother liquor after sodium chlorate crystallization is treated with an aluminum salt, preferably, aluminum chlorohydrate to precipitate a portion of the silicon impurity and heavy metal contamination from the mother liquor.
Accordingly, in one aspect of the present invention, there is provided a means of removing silicon and heavy metals from a mother liquor which is recycled to an electrochemical cell subsequent to cry tion of sodium chlorate and removal thereof A process for the prepration of sodium chlorate comprises (1) electrolyzing an aqueous solution of sodium chloride to form an aqueous solution of sodium chlorate and sodium chloride, (2) crystallizing sodimn chlorate from the aqueous solution of sodium chlorate and sodium chloride to form a mother liquor and sodium chlorate crystals, (3) separaing sodium chlorate crystals from the mother liquor, (4) adding make-up sodium chloride solution to the mother liquor to form a feed solution, and recycling the feed solution to an electrolyzing step. The improvement of the invention comprises treating at least a portion of the mother liquor with an aluminum salt, preferably, aluminum chlorohydrate to precipitate a part of the silicon and heavy metal contamination therein, separating the precipitate thus formed containing a portion of the silicon and heavy metal content of the mother liquor, and subsequently recycling the treated portion of the mother liquor to the electrolyzing step.
A batch or continuous process for the removal of silicon and heavy metal contamination in the mother liquor is contemplated, with a continuous process preferred, in which the rate of removal of the silicon and heavy metal impurity present in the mother liquor is adjusted to match the rate of sodium chlorate formation occuring in the electrochemical cell and the subsequent crystallization of sodium chlorate from the cell liquor to produce the mother liquor. Typically, removal efficiencies of about 50 percent of the silicon present in the mother liquor and higher proportions of removal of the heavy metals are sufficient to maintain the electrical efficiency of the process for sodium chlorate production.